Sorry the "Circle of Life" was cut short for your baby birds.
I live on a lake, and like you, we have some birds (waterfowl) who've become neighborhood mascots, particularly "Mama Duck" who nests up close to a house for protection, and always seems to keep her dozen ducklings safe from harm. For whatever reason, this Spring "Tyson" - a huge domestic goose, decided he was Top Dog - and killed a Mallard drake who was wooing Mama Duck... then Tyson proceeded to, um... well, if you've ever witnessed ducks breeding, it's kind of brutal. Glad Mama Duck wasn't drowned. So Tyson was on a testosterone-fueled rampage.... and then he disappeared. And the big cob Mute Swan who nests about 200' away was seen in the area. All we can figure is that Mr Bada$$ Tyson decided to take on the swan..... and lost.
We call it "Duck Drama"